Add span fields to NumberRangeFormatter
Span fields were added to DateIntervalFormat and ListFormatter. They should also be added to NumberRangeFormatter.
Increasing work level to Days, since it will be interesting to figure out how to get the span indices to work, especially in C++. Also raising priority to Major so that this will be solved in ICU 69 and unblock implementors of the related ECMAScript proposal.
Consider deprecating/removing the old nextFieldPosition and getAllFieldPositions methods in formatted number range along with this ticket.